Here's how I use Google Trend's with Craigslist.
http://webmasterautomation.com/blog/...aigs-list-tip/

First, do a search at
http://www.google.com/trends

It will show you the area’s of the country/world with the most searches,
for your particular keywords. So if it’s Nashville, TN or Atlanta, Ga.
then you would place your ad in that particular city with Craigs List.

Craigs List is a great source for leads and by using Google Trends
to Geo-Target your hottest market you can really increase response
rates.

Of course, you can do this with the other major free classified ad
sites also. It’s not just limited to Craigs List. Do a search of this
blog for “classified ads” and you’ll find a list of the biggest free
classified sites online. I listed them a couple month’s back.